1. Field of the Invention
The present invention relates to a maraging steel that is particularly suitable for the manufacture of parts which must have very good fatigue strength, and to a process for manufacturing a strip or a part cut therefrom.
2. Discussion of the Background
Numerous parts are manufactured from maraging steel strips containing, in % by weight, about 18% of nickel, 9% of cobalt, 5% of molybdenum, 0.5% of titanium and 0.1% of aluminum, and having been treated to achieve an elastic limit of greater than 1800 MPa. These strips are obtained by hot rolling and cold rolling. The strips or parts cut out of the strips are then hardened by hardening heat treatment at around 500xc2x0 C. The parts are surface-nitrided to improve their fatigue strength. Unfortunately, the fatigue strength of these parts is insufficient.
In order to improve the fatigue strength of the parts, it has been proposed to use maraging steels having different chemical compositions and mechanical characteristics, such as maraging steels containing 18% of nickel, 12% of cobalt, 4% of molybdenum, 1.6% of titanium and 0.2% of aluminum, or maraging steels containing 18% of nickel, 3% of molybdenum, 1.4% of titanium and 0.1% of aluminum, or even maraging steels containing 13% of chromium, 8% of nickel, 2% of molybdenum and 1% of aluminum. None of these steels, however, has yielded satisfactory results, the fatigue strengths always being poorer than that of parts manufactured with the standard steel.
One object of the present invention is to remedy these problems and to provide a strip or a part of maraging steel having improved fatigue strength.
Another object of the invention is a process for the manufacture of a strip, or a part cut out of a strip, of cold-rolled maraging steel. According to this process, before a hardening heat treatment is performed, the strip or the part is subjected to cold plastic deformation with a degree of working greater than 30% and the strip or the part is subjected to recrystallization annealing in order to obtain a fine-grained structure with ASTM index higher than 8.